toolchain-external: CodeSourcery NIOSII: support only one version

See the conclusion about external toolchains during the Buildroot
meeting [1]:
"In the future, we stick to a single external toolchain version. The
Kconfig symbol should not encode the version (avoid legacy handling)"

[1] http://elinux.org/index.php?title=Buildroot:DeveloperDaysELCE2015#Report



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>